Cleaning of a 3d printed article

ABSTRACT

The present disclosure relates to a method and apparatus for cleaning a 3D printed article, in particular a 3D printed heat exchanger. After 3D printing, an article may have internal passages formed from bonded powder and said passages may contain unbonded powder that needs to be removed before further use of/processing of the article. To remove this unbonded powder, the article is filled with a cleaning fluid and vibrated. The cleaning fluid is then pumped out of the article and past a sensor that generates a magnetic field. The sensor detects the presence of powder particles in the fluid by detecting a perturbation of the magnetic field caused by said particles. The fluid is then filtered and returned to a reservoir for use. The sensor may indicate the article is sufficiently clean when a detected concentration of particles in the fluid drops below a threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application No. 16,824,940,filed Mar. 20, 2020 which is a division of U.S. application Ser. No.15/591,542 filed May 10, 2017, issued as U.S. Pat. No. 10,634,440 onApr. 28, 2020, which claims priority to United Kingdom PatentApplication No. 1610728.6 filed Jun. 20, 2016, the entire contents ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a method and system for cleaning a 3Dprinted article such as a heat exchanger.

BACKGROUND

3D printing may be used to create three-dimensional objects by printinglayers of material on top of previously laid-down layers of material,such that the finished or partially-finished 3D printed product isformed of a number of thin layers of material. 3D printing is sometimesalternatively known as “additive manufacturing” or “rapid prototyping”,all of which may refer to the general technique of producing 3D articlesfrom printing multiple thin layers of the article, one on top of theother.

One 3D printing technique is to provide a movable stage beneath acomputer-controlled print head. A thin flat layer of powder is spreadover the stage and the printer head sprays a binder material ontospecific points on the layer of powder, in accordance with across-section of the finished 3D printed article. The powder may be oneof a variety of materials, for example a powdered metal or a powderedceramic. The binder may be one of a variety of sprayable products, suchas an adhesive. The binder reacts with the powder to form a solid blobof material at the point where the binder is sprayed. After the printerhead has sprayed all the designated areas of the first layer of powder,the stage may be lowered and a new layer of loose powder is overlaid onthe previous layer (that, after printing, comprises areas of loosepowder and areas of bound powder). The printer head scans over the newlayer of powder and deposits binder in accordance with the instructionsfrom the controller. Thus, a second layer of the final article iscompleted. This process is repeated until all of the layers of thearticle have been printed.

Instead of using a binder from the printer head, a laser or electronbeam may be used to bind the powder together. In this form of 3Dprinting, the laser (or electron beam) melts or welds together the metalparticles caught within the beam. The beam is then scanned over thepowder layer to form that cross-sectional layer of the finished orpartially-finished article.

If a 3D printed article is printed using the above-described method, andthe finished article contains internal passages, those passages may befilled with unbound powder. This powder must be removed to clear thepassages and leave the desired article with internal passages properlyformed.

After removal of the powder, the product may be complete or it mayundergo further processing steps, such as heat treating.

If the powder was a metal powder, it may be desired to sinter the metalparticles together to increase the strength of the article. In thisprocess, the green (i.e. partially-completed) 3D printed article may beplaced in a furnace and heated. Before heating, any excess loose powdershould be removed so that the finished article properly matches thedesired shape that was printed by the 3D printer. This is because aheat-treating step may fuse any unbonded powder that remains in theinternal passages which may undesirably distort the internal passages.If the 3D printing process utilised a binder, then the binder may havebeen selected to thermally decompose during heating to allow the metalpowder particles to sinter together firmly.

One type of article that may be made by 3D printing is a heat exchanger.Heat exchangers typically have a variety of internal passages for afirst fluid (e.g. refrigerant) which passages may themselves surround orpartially surround external passages for a second fluid (e.g. air or asecond refrigerant). When a heat exchanger is constructed through 3Dprinting, there may be many passages, both internal and external,containing loose powder. The loose powder requires a clear access pathto be properly removed, which complete heat exchanger assemblies oftendo not have due to the complex nature of the flow paths that have beendesigned for the ideal thermal performance and flow distribution. Ifthis loose powder is not removed, it may solidify during any subsequentheat treatment and hence block the heat exchanger's flow channelsthereby affecting the performance of the unit. Further, as the internalpassages of the heat exchanger cannot be easily inspected it may bedifficult to determine with confidence whether any cleaning process hasbeen successful in completely removing loose powder.

Thus, there is a need for methods and systems for removing loose powderfrom 3D printed articles, including removing powder from internal pathswithin the green article or the finished article. Various methods forcleaning loose powder from internal passages have been proposed in theprior art.

US 2016/0074940 discloses a method of removing loose powder materialfrom a cavity within a 3D printed part, wherein the cavity has at leastone opening leading to the outside of the part. The part is placed on astage and vibrated until the powder fluidizes and flows out of thecavity via the opening(s).

Such conventional methods have generally been considered satisfactoryfor their intended purpose. However, there is a need in the art forimproved methods of clearing powder from 3D printed articles,particularly heat exchangers.

SUMMARY

According to a first aspect, the disclosure provides a method ofcleaning an article comprising internal channels, the method comprising:pumping a cleaning fluid through the internal channels initiallycontaining particles, wherein the particles have a different magneticpermeability from the cleaning fluid; passing the cleaning fluid past asensor generating a magnetic field; and detecting the presence ofparticles in the fluid by detecting perturbation of the magnetic field,the perturbation being caused by the particles.

The step of detecting may comprise determining a concentration ordensity of particles within the fluid based on a determined degree ofperturbation of the magnetic field.

The sensor may generate a magnetic field by means of an inductive coil;optionally the perturbation of the magnetic field may be detected as achange of a voltage across the inductive coil.

The method may further comprise a step of determining the article isclean when no particles are detected in the fluid or when aconcentration of particles detected in the fluid has dropped below athreshold concentration, optionally wherein the threshold concentrationis 5%, optionally wherein the threshold concentration is 1%.

The method may comprise filtering particles out of the fluid using afilter after the detection step. The clean fluid may be passed back toan inlet of a pump used for pumping the cleaning fluid.

The step of pumping may comprise the steps of: attaching the article toa fluid circuit; placing the article on a stage; pumping cleaning fluidinto the article; vibrating the stage and thereby the article; andpumping the fluid out of the article to the sensor.

Before the step of vibrating, the method may comprise closing valves inthe fluid circuit to constrain the fluid to the internal passages of thearticle during the vibration step.

The method may comprise a step of calibrating the sensor by: passing aclean fluid through the sensor and measuring a first voltage across thesensor; and passing a fluid containing a known concentration ofparticles having known magnetic permeability through the sensor andmeasuring a second voltage across the sensor.

The particles may be comprised of metal or ceramic.

The article may be a 3D printed article. For example, the article may bea heat exchanger. The particles may be residual particles from the 3Dprinting process.

The cleaning fluid may be water or water containing detergent. Thecleaning fluid may optionally comprise an abrasive.

The present disclosure also provides an apparatus for cleaning anarticle comprising internal channels, the apparatus comprising: a pumpconfigured to pump cleaning fluid through the internal channels tothereby remove particles from the internal channels into the fluid; anda sensor arranged to generate a magnetic field; wherein the apparatus isconfigured to detect, in fluid being pumped past the sensor, thepresence of particles in the fluid having a different magneticpermeability than the fluid, by detecting perturbation of the magneticfield caused by the particles.

The apparatus may comprise a fluid circuit including the pump, whereinthe pump defines an upstream position of the fluid circuit and isconfigured to pump a fluid through the circuit; wherein the fluidcircuit comprises a connection downstream of the pump, the connectionbeing configured to attach to an article comprising internal passages;and wherein the sensor is located downstream of the connection.

The apparatus may be configured to determine a density or concentrationof particles within the fluid based on a determined degree ofperturbation of the magnetic field.

The sensor may comprise an inductive coil for generating the magneticfield; optionally wherein the apparatus is arranged to detect a changeof a voltage across the inductive coil from which the perturbation ofthe magnetic field is determined.

The circuit may comprise a stage configured to vibrate the article,preferably with high frequency low amplitude vibrations.

The circuit may comprise a filter downstream of the sensor for filteringparticles out of the fluid.

There may be a controller connected to the sensor, the controllerconfigured to apply a voltage to the induction coil and to detectperturbations of the voltage across the induction coil.

The sensor may comprise one or more indicators for indicating aninstantaneous particle concentration in fluid passing the sensor. Forexample, the one or more indicators may be a series of lights, a seriesof LEDs, or a display screen.

The apparatus may be for cleaning an article produced by 3D printing,preferably a heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present disclosure will now be described ingreater detail by way of example only and with reference to theaccompanying drawings in which:

FIG. 1 shows a computerised tomography scan of a 3D printed heatexchanger;

FIG. 2 shows a system for cleaning loose powder from a 3D printedarticle according to an example of the disclosure;

FIG. 3 shows a sensor for detecting a concentration of particles in afluid according to an example of the disclosure; and

FIG. 4 shows a graph of particle-concentration against time, as detectedby the sensor.

DETAILED DESCRIPTION

The following description will be given with reference to a heatexchanger that has been 3D printed from a metallic powder by lasersintering or electron beam melting (EBM). However, the method and systemaccording to the present disclosure are applicable to other printedarticles, to articles made from different materials, and to other 3Dprinting techniques that leave unwanted powder in the printed article.

FIG. 1 shows a computerised tomography (CAT) scan of a heat exchanger 10(a “plate and pin” heat exchanger) that has been 3D printed by lasersintering of successive layers of loose powder. The heat exchanger 10comprises a plurality of plates 4 held apart by pins 6. Flow channels 2are defined between the plates 4 and around the pins 6. Loose powder 8can be seen in FIG. 1 adhering to the plates 4 and the pins 6. In FIG. 1, the majority of the loose powder 8 that filled the heat exchanger 10immediately after printing has already been removed. Nonetheless, someloose powder 8 may still be seen adhering to the walls of the heatexchanger in FIG. 1 . However, immediately after printing the flowchannels 2 will be completely filled with loose powder. This loosepowder must be removed before the heat exchanger can be used.

With some 3D printed articles, it may be possible to pour out the loosepowder from any internal channels or cavities. With other 3D printedarticles, it may be necessary to provide further mechanical and/orchemical cleaning to remove the loose powder, especially those withcomplex internal passages. Loose powder that is directly adjacent bondedpowder may be more difficult to remove, e.g. due to loose powder stuckamong the rough surface of the bonded-powder of the article.

FIG. 2 shows a system for cleaning loose powder from the internalpassages of a 3D printed article, for example, a heat exchanger 10. Theloose powder comprises a plurality of particles, and the terms “powder”and “particles” are used interchangeably in this specification. Thesystem may comprise a stage 20 upon which the heat exchanger 10 may beplaced. The stage 20 may be configured to vibrate and may be controlledby a controller 30. The stage 20 may therefore be termed a “vibrationtable”. The vibration may be through the use of conventional shakers orultrasonic transducers, or other known methods of applying vibration.

The heat exchanger 10 may be connected into a cleaning circuitcomprising a circuit of pipes 22. Two valves 12 a, 12 b may be disposedon the pipes 22 where they enter/exit the heat exchanger 10.

The system may also comprise, as part of the cleaning circuit and inserial relationship, a sensor 14, a particle filter 18, a fluidreservoir 26 and a pump 24. The sensor 14 may be an in-line particleinspection sensor that provides an indication of when the cleaningprocess has achieved the desired result, as described in greater detailbelow. In FIG. 2 , the pump 24 and fluid reservoir 26 are shown in thesame location. However, the pump 24 may alternatively be providedseparately from the reservoir 26.

The sensor 14 may connect via wires 16 to a computer 28 arranged toprocess data collected via the sensor 14 and received via wires 16, andhaving a power supply for powering the sensor 14. The computer 28 forthe sensor 14 and the controller 30 for the stage 20 may both beprovided within a single computer or may be separate, as shownschematically in FIG. 2 .

The filter 18 is for filtering loose powder 8 that is carried by thefluid flowing through the pipes 22.

The reservoir 26 may contain a cleaning fluid for removing loose powder8 from the heat exchanger 10. The cleaning fluid may be water or may bewater mixed with a detergent, or may be any other suitable fluid forconveying loose powder 8 out of the heat exchanger 10. The cleaningfluid may optionally contain abrasives to assist removal of loosepowder, for example loose powder caught in rough surfaces of the heatexchanger walls.

In operation, the heat exchanger 10 may be removed from the 3D printer,with its flow channels 2 filled with loose powder that is to be removed.When thus filled with powder, the heat exchanger 10 will be referred toas a “green heat exchanger”, to indicate that the product is not yetcompleted (“green” does not refer to the colour of the heat exchanger1). After cleaning, the green heat exchanger will be referred to as “aheat exchanger”.

The green heat exchanger 10 may be placed on the stage 20 and connectedto the pipes 22 of the cleaning fluid circuit with valves 12 a, 12 b oneither side of the green heat exchanger 10. The pump 24 may drawcleaning fluid from the reservoir 26 and pump it into the green heatexchanger 10.

In a first step, the fluid may be pumped into the green heat exchanger10 until fluid first reaches the downstream valve 12 b, i.e. the valve12 b on the far side of the green heat exchanger 10 from the pump 24. Atthis point, both valves 12 a, 12 b may be closed and the pump 24deactivated.

The stage 20 is then vibrated under the direction of the controller 30.The vibration may be low-amplitude high-frequency vibrations. Usinglow-amplitude vibrations may avoid damage to the article being cleaned.The vibration may be at a single frequency or may be varied over a rangeof frequencies, and may be applied for a predetermined duration. Thevibration may loosen the loose particles 8.

The most appropriate frequencies and amplitudes for the cleaning processwill be dependent on the part design, material type and the size ofpowder particles being removed from the part. By way of non-limitingexample, a frequency range of 20-400 kHz may be used. For more robustcomponents and those containing heavier loose particles of powder,frequencies below 80 kHz may be appropriate. For more delicatecomponents and components for which a higher level of cleanliness isrequired, frequencies above 80 kHz may be appropriate. A mix of theseprocesses may be useful, for example starting with low frequencyvibrations to remove larger particles and then switching to highfrequency for the removal of smaller (e.g. submicron) particles.

After the vibration step, the valves 12 a, 12 b may be opened and thepump 24 restarted. The fluid is pumped out of the green heat exchangerthrough pipes 22 along a path A, carrying with it some of the loosepowder 8. The fluid flows though pipes 22 past the sensor 14 and onwardsinto the filter 18 where the powder is partly or entirely filtered outof the fluid. The filtered fluid may then continue to flow through thepipes 22 back into the reservoir 26 ready to be used again. The sensor14 may output a signal indicative of the particle density of loosepowder suspended in the fluid flowing through the sensor. As describedin more detail below, the sensor may thus be used to determine theprogress of the cleaning process and when it is finished, i.e. when thearticle is sufficiently clean.

In one example, after the fluid has completed a single circuit of thesystem (i.e. from the reservoir 26, via the pump 24, green heatexchanger 10, sensor 14, filter 18, and back to the reservoir 26) theprocess may restart, with the pump 24 filling up the green heatexchanger 10, vibrating the heat exchanger, and then pumping the fluidout of the green heat exchanger 10 again. Alternatively, after the firstvibration step, the pump may continuously draw fluid from the reservoirand pump it through the green heat exchanger until the green heatexchanger 10 is deemed to be clean (as described below). In thiscontinuous example, the vibration may be applied continuously,intermittently, or not applied at all after the first vibration stage.

The sensor 14 is shown in more detail in FIG. 3 . The sensor 14 maycomprise an inductive coil (not shown) wrapped around the pipe 22. Thepipe 22 may be made from a non-ferrous material or from a ferrousmaterial. The computer (controller) 28 may provide a voltage to theinductive coil which generates a magnetic field that extends through theportion of the pipe 22 within the coil and thus through the fluid withinthe pipe 22. The particle-containing fluid will perturb (disrupt) themagnetic flux (field) generated by the inductive coil, and thisperturbation can be measured as a change in the voltage across the coil.

Provided that the particles have a different magnetic permeability fromthe fluid, different quantities of particles in the fluid will perturb(disrupt) the magnetic field (when compared to the original magneticfield applied) to different degrees. Thus, the perturbation can be usedas a measure of the quantity of particles present in the fluid, andtherefore how clean it must be.

For example, the voltage across the inductive coil may change dependingupon the concentration of particles 8 within the fluid, this voltage maybe measured and the data provided to the computer 28. The computer maybe programmed to use this measured voltage together with data of themagnetic permeability of the particles and fluid, to thereby estimatethe concentration of particles within the fluid.

The change in magnetic flux in the inductive coil may be empiricallyrelated to the density of particles in the fluid flowing through thesensor. It is anticipated that the change in magnetic flux in theinduction coil will be proportional to the density of particles in thefluid flowing through the sensor:

ΔB∝D _(P) where: B=magnetic flux

D_(P)=density or concentration of particles

The relationship may be determined through experimentation. Varyingparticle densities may be introduced to a fluid flow and thecorresponding magnetic flux changes measured. Plotting these againsteach other may allow the necessary relationship to be extracted from theresulting graph. The relationship may be different depending on whetherthe particles comprise ferrous or non-ferrous material.

The sensor 14 may be calibrated by comparing the sensor's output whenclean fluid (i.e. powder-free) is pumped through the sensor 14, with thesensor's output when fluid containing a known particle concentration ispumped through the sensor 14.

FIG. 4 shows a graph of particle concentration D_(P) within the fluidagainst time T during a cleaning process. The time T may either bemeasured continually, e.g. in seconds, or may be measured in number ofcycles of the cleaning process if discreet steps are used. The units ofD_(P) are arbitrary and may correspond to a density or to aconcentration (and these terms are generally used interchangeably inthis specification). For example, D_(P) may be measured as: the numberof particles per unit volume of fluid; the number of particles per unitmass of fluid; the weight of particles per unit volume of fluid(generally termed “density”); or the weight of particles per unit weightof fluid. Where the units of measurement of the particles match theunits of measurement of the fluid, the result may be expressed as apercentage and termed “concentration”. For example, if there is 1 g ofparticles carried by 100 g of fluid, it may be said that theconcentration of particles is 1%.

The concentration of particles detected by the sensor 14 is shown by aline 50 on the graph. A threshold concentration of particles can bechosen, depending on application, as indication that a sufficient amountof loose powder has been removed from the green heat exchanger 10 by thecleaning process and thus the article is sufficiently clean. After thecleaning process has run for a number of cycles or for a certain time,the detected particle concentration 50 may drop below the thresholdconcentration 52. This is because as cleaning is being performed powderis gradually removed from the heat exchanger, and thus the amount ofloose powder 8 remaining in the heat exchanger 10 reduces, so the amountof loose powder 8 that is carried out with the fluid during furthercleaning reduces. I.e., the maximum possible concentration of particlesin the fluid drops because the particle concentration is now limited bythe amount of remaining loose power 8 in the heat exchanger. Thus, thesystem may be configured to detect when the particle content of thefluid is at or below the threshold concentration, and thereby determinethat the article is now clean. The system may be configured to stop thecleaning process at this point. The threshold concentration of particlesin the fluid below which the article is considered clean may be e.g. 5%,2% or 1%. The concentration (or density) may be measured in any of themanners described above.

The particle concentration Dp in the fluid may be correlated with theamount of loose powder 8 remaining in the heat exchanger 10. Calibrationtests may be performed to find such a correlation. For example, when theparticle concentration Dp drops below the threshold, it may bedetermined that there is a known, limited, amount of loose powder 8remaining in the heat exchanger 10, or in other words that a certainpercentage of the original quantity of powder has been removed. As onenon-limiting example, the threshold concentration of particles in thefluid may be set such that, when the threshold is reached, 90%, or 95%,or 99% of the loose powder within the article has been removed.

The sensor 14 may be calibrated to detect a particular concentration ofparticles within the fluid. For example a first voltage across theinduction coil may be associated with a first particle concentrationwithin the fluid (e.g. a high particle concentration) and a secondvoltage across the induction coil may be associated with a secondparticle concentration within the fluid (e.g. a low particleconcentration) and a third voltage across the induction coil may beassociated with a clean fluid containing substantially no particles fromthe 3D printed article (e.g. the threshold concentration has beenreached).

The sensor 14 may further comprise a visual indicator 15 that indicatesthe detected particle concentration 50 within the fluid. The visualindicator 15 may comprise a series of LEDs having different colours,wherein different LEDs are lit depending on the detected concentration.For example, a red LED may indicate a high concentration 50 of particles8, one or more orange/yellow LEDs may indicate an intermediateconcentration 50 of particles, and a green LED may indicate that aparticle concentration 50 has dropped below the threshold concentration52. The visual indicator 15 may provide a quick indication to anoperator as to the progress of the cleaning process.

A larger difference in the magnetic permeability of the powder 8compared to the cleaning fluid may be more easily detected by the sensor14. This may be the case if the heat exchanger is formed from a powdercontaining Ni, Al, or Fe or a mixture thereof and/or alloys thereof.However, the sensor may be calibrated to other materials, such asnon-ferrous metals and ceramics.

As described in detail above, the method of the present disclosure mayinvolve one or more of the following steps:

-   -   1) Place the 3D printed article on a stage;    -   2) Connect the article into a circuit of pipes;    -   3) Pump cleaning fluid via the pipes into the article until a        desired quantity is provided e.g. the article is full;    -   4) Close valves at the entry and exit of the article and shut        off the pump;    -   5) Vibrate the article on the stage;    -   6) Open the valves and restart the pump;    -   7) Pump the fluid out of the article through the pipes and past        a sensor;    -   8) Detect a concentration of particles within the fluid with the        sensor;    -   9) Filter the particles out of the fluid downstream of the        sensor;    -   10) Return the filtered fluid to a reservoir for the pump.

As described above, the step of detecting a concentration of particlesmay be performed by a sensor comprising an induction coil having avoltage. The magnetic field produced by the induction coil may interactwith, e.g. be perturbed by, the particles in the fluid, provided themagnetic permeability of the particles is different from the magneticpermeability of the fluid. This perturbation may be indicative of theparticle concentration within the fluid and may be detected by thesensor. The sensor may output a signal indicative of a particleconcentration within the fluid.

After the fluid has been returned to the reservoir, the process mayrepeat steps 3 to 10 until a concentration of particles drops below athreshold. The number of cycles required before the article issufficiently clean may depend on the length and configuration of theinternal passages.

Alternatively, the pump may be operated to pump a continuous flow ofcleaning fluid through the article. The reservoir may be provided with asufficient quantity of fluid to enable continuous operation fromstart-up, or continuous operation may not be possible until thereservoir starts to be replenished with returned fluid. During thiscontinuous flow, the vibration may optionally be applied continuously orintermittently or not at all. Continuous flow may continue until aconcentration of particles detected by the sensor drops below athreshold. The amount of time required before the article issufficiently clean may depend on the length and conformation of theinternal passages.

The particle concentration at a given moment may be displayed by one ormore indicators on the sensor. For example, LEDs or an LCD displaydisposed on the sensor.

By providing an indication of the particle concentration in the cleaningfluid, and a determination of when a threshold concentration is reached,the present disclosure thereby enables the user to determine if and whenthe article is sufficiently clean, and thus if and when the cleaningprocess can be stopped. This ensures both that the article is cleanenough for use, and also avoids unnecessary continuation of the cleaningprocess. This is particularly useful with articles such as heatexchangers, of which the internal channels are difficult to inspect,since the system can provide the user with confidence that the cleaningprocess has been successful in removing unfused powder. The method mayprovide an accurate and sophisticated way of measuring particle densityand extent of a cleaning process.

The above description is of specific examples only and it will beappreciated that variations may be made to the embodiments withoutdeparting from the broad scope of the disclosure as defined by thefollowing claims.

1. An apparatus for cleaning an article comprising internal channels,the apparatus comprising: a pump configured to pump cleaning fluidthrough the internal channels to thereby remove particles from theinternal channels into the fluid; and a sensor arranged to generate amagnetic field, wherein the apparatus is configured to detect, in fluidbeing pumped past the sensor, the presence of particles in the fluidhaving a different magnetic permeability than the fluid, by detectingperturbation of the magnetic field caused by the particles; wherein theapparatus comprises a stage configured to vibrate the article; andwherein the stage is configured to vibrate the article at a firstfrequency to loosen particles and vibrate at a second frequency higherthan the first frequency to loosen particles smaller than those loosenedby the first frequency, wherein the first and second frequencies are inthe range of 20 to 400 kHz.
 2. An apparatus as claimed in claim 1,further comprising a fluid circuit including the pump, wherein the pumpdefines an upstream position of the fluid circuit and is configured topump the fluid through the circuit; wherein the fluid circuit comprisesa connection downstream of the pump, the connection being configured tofluidly attach to the article comprising internal passages; and whereinthe sensor is located downstream of the connection.
 3. An apparatus asclaimed in claim 1, wherein the apparatus is configured to determine adensity or concentration of particles within the fluid based on adetermined degree of perturbation of the magnetic field.
 4. An apparatusas claimed in claim 3, wherein the apparatus is configured to determinethe article to be clean when no particles are detected in the fluid orwhen a concentration of particles detected in the fluid is below athreshold concentration.
 5. An apparatus as claimed in claim 4, whereinthe threshold concentration is 5%.
 6. An apparatus as claimed in claim1, wherein the sensor comprises an inductive coil for generating themagnetic field.
 7. An apparatus as claimed in claim 6, wherein theapparatus is arranged to detect a change of a voltage across theinductive coil from which the perturbation of the magnetic field isdetermined.
 8. An apparatus as claimed in claim 2, wherein the fluidcircuit comprises valves configured to constrain fluid to the internalchannels of the article when the article is vibrated on the stage.
 9. Anapparatus as claimed in claim 8, wherein the fluid circuit comprises acircuit of pipes and the valves are disposed on the pipes.
 10. Anapparatus as claimed in claim 9, wherein a valve is disposed on a pipeat a point where it is configured to connect to the article and supplyfluid to enter the article and wherein a valve is disposed on anotherpipe at a point where it is configured to connect to the article andreceive fluid exiting the article.
 11. An apparatus as claimed in claim1, wherein the stage is configured to vibrate the article with highfrequency low amplitude vibrations.
 12. An apparatus as claimed in claim2, wherein the circuit comprises a filter downstream of the sensor forfiltering particles out of the fluid.
 13. An apparatus as claimed inclaim 7, further comprising a controller connected to the sensor, thecontroller configured to apply a voltage to the inductive coil and todetect perturbations of the voltage across the inductive coil.
 14. Anapparatus as claimed in claim 1, wherein the sensor comprises one ormore indicators for indicating an instantaneous particle concentrationin fluid passing the sensor.
 15. An apparatus as claimed in claim 14,wherein the one or more indicators are a series of lights, a series ofLEDs, or a display screen.
 16. An apparatus as claimed in claim 1,wherein the article is a three-dimensional printed heat exchanger. 17.An apparatus as claimed in claim 16, wherein the particles are residualparticles from the 3D printing process.
 18. An apparatus as claimed inclaim 1, wherein the first frequency is a frequency below 80 kHz, andthe second frequency is a frequency above 80 kHz.